Case Study Eagle Project, Michigan · Case Study Eagle Project, Michigan. STRATUS CONSULTING...
Transcript of Case Study Eagle Project, Michigan · Case Study Eagle Project, Michigan. STRATUS CONSULTING...
STRATUS CONSULTING Kuipers & Associates, LLC
Case StudyEagle Project, Michigan
STRATUS CONSULTING Kuipers & Associates, LLC
Map of the Eagle Project location and surrounding area
Source: Foth & Van Dyke and Associates, 2006a, Figure 2-1.
STRATUS CONSULTING Kuipers & Associates, LLC
Ore and host rock overview High grade nickel-copper sulfide deposit Main ore minerals are pentlandite (nickel sulfide mineral) and chalcopyrite
(a copper-iron sulfide mineral) in pyrrhotite (iron sulfide) Massive sulfide unit (MSU)
– Ore, known to produce acid drainage– 50 to 100% sulfide (~32-38% sulfur)
Semi-massive sulfide unit (SMSU)– Ore, known to produce acid drainage– 30 to 50% sulfide (~12-15% sulfur)
Peridotite/intrusive unit– Alternatively considered ore and non-ore; development rock– Up to 30% sulfide, mostly acid generating
Country rock/sedimentary units– Non-ore, host rock; development rock– 0.2 to 1.4% sulfur, mostly acid generating
High sulfide content of underground workings – cement backfilling unproven technology
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Geochemical characteristics and testing of deposit
Rock type/geochemical unit %S or sulfide in unit
Acid generation potential
summarya
Number of kinetic tests
run %S of samples for kinetic testsSedimentary units (sandstone/siltstone/ hornfels)
0.2-1.4%S 69% AG; 11% uncertain; 20% non-AG
Siltstone: 5Sandstone: 1Hornfels: 1
0.31 to 2.1%S
Peridotite/disseminated sulfide unit/ pyroxenite (along margins of the intrusions, above and below the upper sulfide zone and above the lower sulfide zone)
3-15% sulfide (disseminated sulfide)Disseminated sulfide =
< 30% sulfide
61% AG; 16% uncertain; 23% non-AG
Peridotite: 4Pyroxenite: 1
Peridotite: 0.2 and 2.44%SPyroxenite: 0.99%S
Massive sulfide unit (MSU) > 80% sulfide50-100% sulfide
32-38%S
3/3 Phase I samples AG 1 36.1%S
Semi-massive sulfide unit (SMSU) 30-50% sulfide12-15%S
3/3 Phase I samples AG 2 12.9 and 8.13%S
a. Using 3:1 NP:AP and Sobek method.AG = acid-generating; S = sulfur.%S in most common sulfides (pyrrhotite, pentlandite, chalcopyrite) ranges from 33 to 42%.Sources: Geochimica, 2004 (for sulfur percentages); Kennecott Exploration, 2005 (for sulfide percentages).
Stratus Consulting, 2007
STRATUS CONSULTING Kuipers & Associates, LLC
Cross-section through crown pillar and ore body
Source: Enlarged from Kennecott Exploration, 2005.
383m
327.5m
SMSU
MSU
MSU=red; SMSU=tan; peridotite=magenta; sedimentary=green
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Comparison of recommended input leachate values (mg/L) for water quality prediction after mining
Source: Humidity cell testing results, Geochimica, 2004.
Stratus Consulting, 2007 Geochimica, 2006Source rock/type mineralization CR Intrusives SMSU MSU Low-S peridotite
High-S peridotite SMSU MSU
SO4 161 135 585 474 3.09 57 426 368Al 5.51 0.0069 0.01 0.005 0.005 0.0078 0.019 0.005Cd 0.0126 0.000025 0.006 0.00085 0.000025 0.000025 0.003 0.00085Co 0.245 0.0029 1.4 0.363 0.00005 0.00016 0.721 0.363Cu 12.7 0.0112 0.0208 0.0048 0.0016 0.005 0.017 0.0048Fe 26.4 0.015 2.16 89.3 0.015 0.015 0.36 89.3Pb 0.15 0.00012 0.0005 0.002 0.00005 0.00005 0.00005 0.00005Ni 2.66 0.754 120 39.9 0.0023 0.0066 68.2 39.9Zn 1.22 0.0029 0.074 0.0127 0.0014 0.001 0.06 0.0127
CR = country rock; SMSU = semi -massive sulfide unit; MSU = massive sulfide unit; S = sulfur.
Stratus Consulting, 2007
STRATUS CONSULTING Kuipers & Associates, LLC
Comparison of HCT Values
Country Rock
02468
101214161820
0 50 100 150 200
Cu
(mg/
L)
Kennecott
Stratus
Peridotite
0
50
100
150
200
0 50 100 150 200
SO4 (
mg/
L)
SO4
Modified from Stratus Consulting, 2007
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Geochimica’s general approach to modeling
HCT concentrations converted to leach rates per unit surface area per unit time
Total surface area of exposed rock in the mine (of each rock type) and the development rock used as backfill were used to compute the mass leached
Mass of leached contaminants was divided by the total water inflow volume
End = concentrations in backfilled underground mine after mining
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Kennecott’s modeling assumptions that minimize predicted mine water concentrations
Mining cannot surgically remove ore– Kennecott: Essentially no ore will remain in mine or development
rock– We used 5% SMSU, 45% intrusives, 45% country rock for
development rock; 12% SMSU (vs. 8%), 3% MSU (vs. 2%), 65% intrusives, 20% country rock for mine wall rock
Development rock contains small particles, and these control leaching– Kennecott: 100% are 10 cm in diameter– We used 90% at 10 cm, 10% at 1 cm diameter
Water infiltrating through the crown pillar will contain contaminants– Kennecott: No contaminated infiltration will move through the
crown pillar to the mine during or after mining– We estimated leachate concentrations using HCT results
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Kennecott’s modeling assumptions that minimize predicted mine water concentrations (cont.)
End of mining (year 7+) leachate concentrations should be represented by later HCT results– Kennecott: HCT results from weeks 20 to 50 are
representative of concentrations after mining– We used HCT results from weeks 50 and 70
Groundwater infiltration to the mine is estimated at 75 gpm– Kennecott: The amount of water entering the mine
will be 180 gpm– We used 75 gpm (expected groundwater inflow
rate used in water balances for MPA)
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Comparison of predicted development rock stockpile water quality using different inputs and assumptions
Geochimica (2005b)Stratus Consulting (2007)
Sulfate 575 5,940Nickel 8.33 102TDS 956 8,340Aluminum 3.46 79.8Beryllium 0.0019 0.051Cadmium 0.0002 0.185Cobalt 0.0008 4.14Copper 5.58 184Iron 26.8 383Lead 0.0004 2.17Zinc 1.90 17.7
TDS = total dissolved solids; Stratus, 2007 = 90% 10cm, 10% 1cm, 5% SMSUNote: Does not include limestone
Stratus Consulting, 2007
STRATUS CONSULTING Kuipers & Associates, LLC
Comparison of predicted mine water quality at the end of mining to relevant standards
Units
Stratus Consulting
(75 gpm inflow)
Geochimicaa
(180 gpm inflow)
Part 201
Part 22 standards MCL SMCL MCLG
TDS mg/L 561 – 500Aluminum g/L 4,950 4.0 300 150 50 to
200Beryllium g/L 3.22 – 4 2 4 4
Cadmium g/L 11.9 0.08 5 2.5 5 5
Cobalt g/L 362 18 40 20
Copper g/L 11,400 2.1 1,400 500 1,300 (TT)
1,000
Lead mg/L 135 0.03 4 2 15 (TT) 0Nickel g/L 15,500 1,770 100 50
Sulfate mg/L 394 28 250 250 250MCL = maximum contaminant level; SMCL = Secondary MCL; MCLG: MCL Goal. TT = Treatment technique. See U.S. EPA, 2007. a. Geochimica, 2006, Table 2; - = not estimated.
Stratus Consulting, 2007
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Overall conclusions
The ore and host rock will produce acidic, metal-rich drainage quickly (during mining) that will last for long periods of time
Leaching of contaminants in the crown pillar was ignored, and water contaminant levels during and after mining were underestimated in Kennecott’s modeling
We predict that concentrations of aluminum, boron, cadmium, cobalt, copper, iron, lead, nickel, nitrate, sulfate, and TDS will exceed Michigan water quality standards after mining; Kennecott predicts only nickel and iron will exceed
More realistic higher concentrations require redesign of water treatment facility and consideration of impacts to groundwater and surface water downgradient of mine and TWIS